Grueretal



Ag. 11, 1959 M. GRUER ET AL AVARIABLE RESISTORS WITH INFINITE RESOLUTION 2 Sheets-Sheet l Filed April 7, 1958 Aug. 11, 1959 M. GRUER ETAL 2,899,661

VARIABLE REsIsToRs WITH INFINITE RESOLUTION i Filed April 7, 1958 2 Sheets-Sheet 2 Q0 3'8/2'0 46 lNvENToRs MORRIS' GRUER 43 O--E /8 WENTHAL Y 74a l [L39 ATTORNEY nited States Patent O f' VARIABLE RESIST ORS WITH INFINITE RESOLUTION Morris Gruer, Brooklyn, and Gary Rosenthal, New York, N.Y., assignors to Eastern Precision Resistor Corp., Brooklyn, NX., a corporation of New York Application April 7, 195s, serial No. 726,188

14 claims. (c1. 33s-144) This invention relates to variable resistors, particularly, but not exclusively, of the type adapted for use in trimming potentiometers.

Variable resistors can be classified into two general categories, to wit, (l) where a wiper arm is caused to engage points, within a single contact plane, on successive convolutions of a resistance wire, and (2) where a :wiper arm is caused to slide continuously along a resistance wire. The first of said categories may be generally referred to as providing stepped variations in resistance, and the second as providing infinite resolution, that is, an innite number of variations in resistance within the physical limitations of the resistance element.

The basic shortcoming of variable resistors in the said stepped category is that they are not adapted for use where an exceedingly high degree of accuracy in attaining a predetermined resistance is required. In other words, in such devices there are a nite number of resistance values which are limited to the number of convolutions of the coil, so that iii-between resistance values cannot be achieved. Moreover, in the conventional construction of stepped variable resistors the wiper arm frequently contacts more than one convolution at one time, thereby making it virtually impossible, with such a structure, to locate a specic point on the wire for a specific resistance.

In the conventional infinite resolution type of variable resistor a continuous length of resistance wire is cemented or otherwise anchored either to the inside or outside of a mandrel, a wiper arm being provided which is adapted for continuous contact with the wire along its entire length. In such constructions either the arm is rotated in a helical path so as to slide continuously along the resistance wire, or the mandrel is rotated while the arm is caused to move in a linear direction in synchronized relation to the helical wire path. While this type of construction of variable resistor is capable of achieving infinite resistance resolution, the complex structure necessary to permit the operative movement of the wiper arm relative to the helically wound wire, or vice versa, makes it necessary that the device have a number of intricate moving parts, and that it be relatively large. Moreover, in such a device the wire must be relatively heavy in order for the wiper to maintain the desired contact, so that a considerable length of wire must be employed in order to obtain the predetennined total resistance value, thereby fu-rther adding to the requirement of large mandrel proportions. Such structures are thus bulky, difficult to manufacture and correspondingly expensive.

It is the primary object of our invention to provide a variable resistor having the advantages of cost and compactness of the stepped type of resistor, and at the same time having the feature of infinite resistance resolution without the aforesaid shortcomings of conventional innite resolution variable resistors. And in the accomplishment of this objective it is a further object of this invention to provide an infinite resolution resistor adapted Patented Aug. 11, 195,9

ICC

for use as a potentiometer, and one that is simple in design, rugged in construction, with a minimum of parts and sufciently small in size as to enable it to have the wide range of uses to which only the stepped type of Variable resistor has heretofore been adaptable.

It is another object of our invention to provide a construction of variable resistor in which a resistance wire is employed in coiled configuration with practically no possibility of a shorting of the adjacent convolutions, and in which a wiper element is employed for contacting onlyr selected points of the wire, Iwithout bridging adjacent convolutions, thereby enabling the obtaining of accurate predetermined resistance values.

Still another object of our invention is `the provision of a construction in which the wiper element and resistance coil are completely sealed from the atmosphere, in contradistinction to the conventional type of variable resistor having a slotted casing through 'which a manually manipulable nger extends, and through which there is communication between the atmosphere and the interior of the device. And in the accomplishment of this object of our invention it is our objective to provide a fully encapsulated variable resistor with an external conveniently positioned manually manipulable control member having a rotary action.

Other objects, features and advantages will appear from the drawings and the description hereinafter given.

Referring to the drawings,

Figure l is an axial section taken through the variable resistor according to an embodiment of our invention', portions being shown in elevation for clarity.

Figure 2 is a section of Figure l taken along l-ine 2 2.

Figure 3 is a section of Figure l taken substantially along line 3 3, a fragment being shown in elevation, a portion being removed for clarity.

Figure 4 is a section of Figure l taken along line 4 4.

Figure 5 is a section of Figure l taken along line 5 5.

Figure 6 is a section of Figure l taken along line 6 6.

Figure 7 is a section of Figure l taken along line 7 7.

Figure 8 is an enlarged fragmentary elevational View showing the mandrel and wiper member in a selected operative position, and illustrating the relative positions of the contact member and the elevated portion of the resistance wire.

Figure 9 is a view like Figure 8, but showing thewiper member and associated parts in another relative position with respect to the mandrel.

Figure l0 is a section of Figure 8 taken along line 10 10.

Figure ll is a fragmentary axial sectional View of one end of the resistor, showing the wiper member in an extreme end position.

Figure l2 is a sectional view of Figure ll taken substantially along line 12 12.

Figure 13 is a schematic diagram showing the electrical circuit of the device of Figires l-l2.

Figure 14 is a fragmentary axial section, partly in elevation, of a variable resistor in accordance with our invention, showing a modied form of the wiper member.

Figure l5 is a section of Figure 14 taken along line 15 15.

The particular form of our invention illustrated in Figures 1 13 is applied to a potentiometer having an outer metal casing 15, of substantially rectangular crosssection, within which is disposed an inner metal housi-ng 16 of similar configuration. The opposite end portions of said housing overlie and are supported by the inwardly extending bosses 17 and 18 of the respective front and rear coacting shaft bearing members 19 and 20 made of insulating material. Extending through the axial apertured portion 21 of said front bearing member 19 is the front shaft 22; and extending through the axial apertured portion 23 of the rear bearing member 20 is the rear shaft 2lisaid shafts 22 and 24 being rotatably supported by said respective apertured portions 21 and 23. Mounted upon the inner terminal portions 25 and 26 of said respective shafts 22 and 24 is the externally threaded-mandrel 27, the opposite end portions of said mandrel having recessed portions 28 and 29 proportioned for receiving and frictionally holding said respective shaft terminals 25 and 26, so that upon an operative rotation of shaft 22, the said mandrel 27 and said rear shaft 24 are also operatively rotated.

Mounted over said mandrel 27 and in threaded engagement therewith is the internally threaded carriage Ynut or wiper member 36 of insulating materiaLsaid wiper member' containing contacting means, whereby said member 30 serves the purpose of a `contact member for the devices resistance wire, in a manner to be hereinafter set forth. For the present it need merely be noted that said wiper member 3ft contains a wire receiving slot 31 and a guide slot 32, both slots extending partially inwardly from the outer periphery of the wiper and in the direction of the longitudinal axis of the mandrel 27. Extending into and in slidable engagement with the slot 32 is the longitudinally disposed guide-bar which, in the form illustrated, consists of an inwardly extending portion `of the lateral wall forming the said inner housing 16. (See Figure 6.) The arrangement is hence such that upon an operative rotation of the mandrel 27, the said wiper member 30, being held against rotation by said guide-bar 33, will move linearly forwardly or rearwardly along the mandrel 27, depending upon the latters direction of rotation.

In the embodiment illustrated, the said wiper member 30 contains a slot 34 intersecting said slot 31, preferably at right angles thereto. Fixedly secured within said slotted portion, preferably by a plastic adhesive (not shown), is the wire contact member 35 consisting, in the particular form shown, of a straight medial portion 36 and hooked terminal portions 37 and 38 in slidable engagement with the respective inner surfaces of the upper and lower walls 39 and 4t) of the said inner housing 16. The said medial portion 36 of said contact element 35 is so proportioned as to occupy only the innermost portion of slot 31, the outermost surface 41 of said medial portion being in spaced relation to the outermost edge 42 of slot 31, thereby leaving room for the operative positioning of the potentiometers resistance wire, as will be hereinbelow explained.

The said resistance wire, generally designated by the reference numeral 43, is of a predetermined gauge proportioned to fit within the helical groove of the threaded portion 44 of the mandrel 27, so that the wire may be operatively wound into said groove. Said wire is anchored at opposite ends of mandrel 27, and extends through substantially all the convolutions of the helical groove except in the region covered by and immediately adjacent the internally threaded wiper 30-the resistance Wire in that region being raised and extending slidably through slot 31, over and in slidable engagement with the said medial portion 36 of the contact element 35, all in a manner to be more fully hereinafter described.

The said front and rear shafts 22 and 24, both of conducting material, carry intermediate the ends thereof the flanges 45 and 46, respectively, these flanges being at the respective front and rear ends of the mandrel 27. In the preferred form of our invention, the peripheral portions of said flanges are of concave configuration, proportioned to receive circumferentially therein the respective opposite end portions of the said resistance wire 43.V In operatively assembling the resistance wire in place, one end portion, such as 47, is wound around a portion of the periphery of flange 46, and. soldered into place. The length of wire is then wound into the helical groove of threaded portion 44 of the mandrel 27. When the Wire reaches the Wiper member 30, it is lifted from the con- 4 i volution 48 adjacent member 30 and passed forwardl through the slot 31 thereof, and thereafter deposited in the convolution 49 in front of and adjacent to the said wiper member. The progressive winding of the resistance wire 43 is then continued7 in the manner aforesaid, until the forward end portion 50 thereof reaches the flange 45, said end portion being then partially wrapped around the concave periphery of said latter flange and soldered into place.

The arrangement is hence such that as the wiper member 36 operatively moves forwardly or rearwardly, it progressively and uninterruptedly engages successive portions of the resistance wire 43, the leading portion of the wiper lifting up the successively engaged portions of the resistance wire from the adjacent portions of the threaded groove and causing the elevated wire portions to travel within said slot 31 and slidably engage the medial portion 36 of said contact element 35. (See Figures Sand 9 showing two different operative positions of the wiper member 30.) It is thus evident that the wiper 30, during its linear travelforwardly or rearwardly, makes continnous contact with the resistance wire from one end to the opposite end. Since such contact is, as aforesaid, continuous in character, rather than step-by-step or intermittent, the device is adapted to provide infinite resolution in a variable resistor, as will more clearly appear from the 'continued description hereinafter given.

The forward portion of front bearing member 19 is provided with a boss 5l over which is mounted the metal contact housing 52, the forward portion shaft 22 extending through the axial opening 53 in the front wall 54 of said latter housing. The lateral wall 55 of said housing contains the two longitudinally disposed transversely spaced apertures 56 and 57 (see Figure 4) through which extend the two arms 58 and 59 of the substantially U- shaped shaft contact member 6i). The said arms 58 and 59 flank and are in yieldable engagement with said shaft 22, so that upon an operative rotation of said shaft said arms serve as wiping contact members. Connecting said arms 58 and 59 is the crown 61 to which is electrically connected, such as by soldering, the conductor 62. The said conductor passes through the recess 63 in the flanged wall 64 of bearing member 19; and it then extends rearwardly between the outer casing 15 and the inner housing 16 to pass through the recess 65 of the flanged wall 66 of the bearing member 20, and outwardly through the rear end cap 67 in the form of lead 62a.

The rear bearing member 20 contains a rearwardly extending boss 68 over which is mounted the rear contact bushing 69 of conducting material; :and into the central hollow portion 70 of `said bushing the terminal portion of said rear shaft 24 extends. In the form illustrated, the reannost end 71 of shaft 24 is spaced from the rear wall 72 of hollow portion 70, providing a chamber which is lled with mercury 73. Electrically connected to the outer surface of said bushing 69 is the conductor 74 which extends rearwardly out through end cap 67 to form conductor lead 74a.

ElectricallyV connected at the outer surface 76 of the inner housing member 16 is the conductor 77, extending rearwardly between outer casing 15 'and housing 16 and through recess 75 of lthe flanged wall 66 of rear bearing member 20-said conductor 77 continuing rearwardly and passing through the end cap 67 as lead 77a.

The arrangement is hence such that the contact element 35 is electrically connected through the metallic housing 16 and through conductor 77 to lead 77a. The forward end 50 of the resistance Wire 43 is connected through the ange 45, front shaft 22, contact member 60 and conductor 62 to the lead 62a. And the rear end 47 of the resistance wire 43 is electrically connected through the ange 46,the rear shaft 24, the mercury 73, the bushing 69 and the conductor 74 to the lead 74a.

Inasmuch as the said contact element 35 is continuously in engagement with the resistance wire 43, it is apparent that said resistance Wire is also electrically connected through said contact element, at regions intermediate its ends, with lead 7 7a.

The circuit is accordingly that of a conventional potentiometer as illustrated in Figure 13, in which the resistance wire, the leads and the wiper member are identiied by their corresponding reference numerals above mentioned.

The forward portion of front shaft 22 extends through the axial opening in the insulating bushing 78 which is force-fitted into engagement with the adjusting cap 79, the latter having the screwdriver slot 80 at the front end thereof. The foremost portion 81 of said front shaft 22 extends into the interior 82 of cap 79, the said interior containing a plastic filler 83. In the form illustrated, said cap 79 contains a ange 84 abutting the inner surface of the front end cap 85 rotatably supporting said cap. The respective front and rear chambers 86 and 87 are lled with a suitable plastic material, such as :a thermosetting resin. It will further be observed that the opposite walls 88 and 89 of outer casing 15 contain, adjacent the respective front and rear ends of said casing, the pairs of apertures 90 and 91, to accommodate suitable eyelets 94 for mounting purposes, in conventional manner.

In the operation of the above-described device, it is thus merely necessary to rotate the cap 79, either manually or by the insertion of a tool in slot 80. Such rotation will cause the front shaft 22, and consequently the mandrel 27 and rear shaft 24 to rotate. And, as aforesaid, since the wiper member 30 is held against rotation by the guide-bar 33, said member will be restricted to linear movement, slidably and progressively engaging successive portions of the resistance Wire 43, thereby enabling the potentiometer to be adjustably trimmed to any desired resistance values within the limitations of the entire length of the resistance coil 43.

It will be observed that since the resistance wire 43 extends transversely across the thin medial portion 36 of the contact element 35, the wiping action of the contact element occurs only at what may be regarded as specific points along the resistance wire. The arrangement is hence such that extremely accurate predetermined resistances can be obtained, for any desired value from zero to the maximum value possible with the coil.

In the embodiment illustrated, the wiper member 30 has its extreme forward and rear positions limited by the respective inner bosses 17 and 18 of the bearing members 19 and 20, respectively, such bosses serving as stops. As will be observed from Figure l1, showing the wiper member against boss abutment 1S, there are still some convolutions 92 rat the end of mandrel 27 that are not in engagement with the contact element 35. The extreme stopped positions of the wiper member 30 short of the extreme ends of the coil of resistance wire 43 eliminate the danger of an overwinding action and consequently of a dislodging of the wires end portions 47 and 50 from their respective anges 46 and 4S. It is preferred, in view of this construction, that the terminal convolutions 92 be shorted, or plated with a low resistance coating 93 of metal, like copper. This will allow resistance adjustment from substantially Zero ohms to the maximum that is possible with the selected coil of resistance wire 43.

In the modification illustrated in Figures 14 and 15, the wiper member 30a is made of a conducting material, such as brass, and is electrically connected with wall 40 of inner housing 16 through the contact element 35a attached to the outer surface of the wiper member. The resistance wire 43 is, in this form of our invention, in direct engagement with the metallic slotted portion 31a. This construction provides substantially the same electrical connection with a lead, such as 77a, without hav- 6 ing the resistance wire come into direct engagement with a contact element, such as 35.

It is thus evident that in both forms of our invention illustrated, infinite resolution results can be obtained in a device which is relatively simple in construction and adapted for fabrication into sizes sufficiently small to enable it to be employed for a great variety of uses, such as have heretofore been possible only with the stepped type of variable resistor. In both of said forms of our invention, as well as in any other form in which the basic elements thereof are employed, contact is made between the Wiper element and the resistance coil in a completely sealed chamber, thereby eliminating the disadvantages of the slotted type of resistor the accurate operation of which is subject to interference by the infiltration of dust and variations in atmospheric humidity.

It is further important to note that even extended periods of use will not aect the accuracy of the device. For example, if it were assumed that as a result of continued use a portion of the resistance wire were to become somewhat abraded, any resulting ditference in resistance valuewhich would necessarily be exceedingly slightcould be offset by changing the relative positioning of the wiper along the resistance wire a sufficiently small amount to correspond to the small resistance change.

In the above description, the invention has been disclosed merely by way of example and in preferred manner; but obviously many variations and modifications may he made therein. It is to be understood, therefore, that the invention is not limited to any specific form or manner of practicing same, except insofar as such limitations are specitied in the appended claims.

We claim:

l. In a Variable resistor, an externally threaded mandrel member of insulating material, a centrally apertured internally threaded conductor wiper member mounted upon and in threaded engagement with said mandrel, means to rotate one of said members relative to the other, whereby said wiper member will move longitudinally relative to said mandrel, a continuous length of resistance wire releasably wound upon Said mandrel member within portions of the threaded groove thereof in flanking relation to said wiper member, a portion of said wire being raised above the mandrel and disposed over and in slidable engagement with a portion of said wiper member, whereby successive portions of said wire will be progressively elevated by the underlying portion of said wiper member during the latters operative movement relative to said mandrel, and conductors electrically connected to said resistance wire and said wiper member, respectively.

2. In a variable resistor, the combination according to claim 1, said conductor wiper member having a recessed portion extending in the general longitudinal direction of said mandrel member, said elevated portion of the wire slidably extending through said recessed portion.

3. In a variable resistor, the combination according to claim 1, said conductor wiper member having a slotted portion extending inwardly from the periphery thereof, the longitudinal extent of said slotted portion being in the general longitudinal direction of said mandrel member, said elevated portion of the wire slidably extending Within said slotted portion.

4. In a variable resistor, the combination according to claim 1, said conductor wiper member having a body portion of insulating material and a contact element of conducting material, said body portion having a slotted portion extending inwardly from the periphery thereof and extending between the front and rear ends of said body portion, said contact element having a portion disposed within said slotted portion, said raised portion of the resistance wire being in overlying slidable engagement with said contact element, the said conductor convnected to the wiper member being in slidable vengagement with said contact element.

5. In a variable resistor, the combination according to claim 1, said conductor wiper member having a body portion of insulating material and a contact element of conducting material having a contact terminal and a supporting portion joined to said terminal, said body portion having a slotted portion extending inwardly from the periphery thereof and extending between the front and rear ends of said body portion, said supporting portion of the contact member being disposed within said slotted portion, said raised portion of the resistance wire being in overlying slidable engagement with said supporting portion and at least partly disposed within said slotted portion, the said conductor connected to the wiper member being in slidable engagement with said terminal of the contact member.

6. In a variable resistor, the combination according to claim 1, said conductor wiper member being of conducting material and having a recessed portion extending in the general longitudinal direction of said mandrel member, said elevated portion of the Wire slidably extending through said recessed portion.

7. In a variable resistor, an externally threaded mandrel member of insulating material, a centrally apertured internally threaded conductor wiper member mounted upon and in threaded engagement with said mandrel, mounting means rotatably supporting said mandrel, means to hold said wiper member against rotation, whereby upon an operative rotation of said mandrel the wiper member will move longitudinally relative to the mandrel, a continuous length of resistance wire releasably wound upon said mandrel member within portions of the threaded groove thereof in flanking relation to said wiper member, a portion of said wire being raised above the mandrel and disposed over and in slidable engagement with a portion of said wiper member, whereby successive portions of said wire will be progressively elevated by the underlying portion of said wiper member during the lattens operative movement relative to said mandrel, and conductors electrically connected to said resistance wire and said wiper member, respectively.

8. In a variable resistor, an externally threaded mandrel member of insulating material, a centrally apertured internally threaded conductor wiper member mounted upon and in threaded engagement with said mandrel, mounting means rotatably supporting said mandrel, said w1per member having a guide slot therein, a guide bar extending longitudinally of the mandrel and being in slidable engagement with said guide slot, whereby said wiper member will be held against rotation and will move longitudinally relative to said mandrel upon an opera t1ve rotation of the mandrel, a continuous length of resistance wire releasably wound upon said mandrel member within portions of the threaded groove thereof in flanking relation to said wiper member, a portion of said wire being raised above the mandrel and disposed over and in slidable engagement with a portion of said wiper member, whereby successive portions of said wire will be progressively elevated by the underlying portion of said wiper member during the latters operative movement relative to said mandrel, and conductors electrically connected to said resistance wire and said wiper member, respectively.

9. In a variable resistor, an externally threaded mandrel member of insulating material, ya centrally apertured internally threaded conductor wiper member mounted upon and in threaded engagement with said mandrel, mounting eans rotatably supporting said mandrel, means to hold said wiper member against rotation, whereby uponan operative rotation of said mandrel the wiper member will move longitudinally relative to the mandrel, a continuous length of resistance wire releasably wound upon said mandrel member within portions of the threaded groove thereof in flanking relation to said wiper member,

a portion of said wire being raised above'the'mandrel and disposed over and in slidable engagement with a portion 'of said wiper member, whereby successive portions of said wire will be progressively elevated by the Yunderlying portion of said wiper member during the latters operative movement relative to said mandrel,'wire anchoring means Vat opposite ends of said mandrel, the opposite terminals of said Vresistance wire being attached to said anchoring means, stop means disposed inwardly relative`to'said anchoring means and positioned for -abutting engagement with said wiper member when it is operatively moved to said respective opposite ends of the mandrel, and conductors electrically connected to said resistance wire and said wiper member, respectively. Y

l0. In a variable resistor, an externally threaded mandrel member of insulating material, a centrally apertured internally threaded conductor wiper member mounted upon and in threaded engagement with said mandrel,Y a housing for said mandrel and wiper member, said housing having a conducting portion extending longitudinally thereof, mounting means rotatably supporting said mandrel, means to hold said wiper member against rotation, whereby upon an operative rotation of said mandrel the wiper member will move longitudinally relative to the mandrel, a continuous length of resistance wire releasably Wound upon said mandrel member within portions of the threaded groove thereof in flanking relation to said wiper member, a portion of said wire being raised above the mandrel and disposed over and in slidable engagement with a portion of said wiper member, whereby successive portions of said wire will be progressively elevated by the underlying portion of said wiper member during the latters operative movement relative to said mandrel, said wiper member having a portion thereof in slidable engagement with said conducting portion of said housing, and conductors electrically connected to said resistance wire and said conducting portion, respectively.

1l. In a variable resistor, an externally threaded mandrel member of insulating material, a centrally apertured internally threaded conductor wiper member mounted upon and in threaded engagement with said mandrel, a housing for said mandrel and wiper member, said housing having a conducting portion extending longitudinally thereof, mounting means rotatably supporting said mandrel, said mounting means comprising bearing members disposed at opposite ends of said housing and having respective inwardly extending portions, means to hold said wiper member against rotation, whereby upon an operative rotation of said mandrel the wiper member will move longitudinally relative to the mandrel, a continuous length of resistance wire releasably wound upon said mandrel member within portions of the threaded groove thereof in flanking relation to said wiper member, a portion of said wire being raised above the mandrel and disposed over and in slidable engagement with a portion of said wiper member, whereby successive portions of said wire will be. progressively elevated by the underlying portion of said wiper member during the latters operative movement relative to said mandrel, wire anchoring means at opposite ends of said mandrel, the opposite terminals of said resistance wire being attached to said anchoring means, the said inwardly extending portions of said bearing members being disposed inwardly relative to said anchoring means and positioned for abutting engagement with said wiper member when it is operatively moved t0 said respective opposite ends of the mandrel, said housing having a conducting portion extending longitudinally thereof, and conductors electrically connected to said resistance wire and said conducting portion, respectively,

l2. ln a variable resistor, an externally threaded mandrel member of insulating material, a centrally apertured internally threaded conductor wiper member mounted upon and in threaded engagement with said mandrel, a housing for said mandrel and wiper member, said housing having a conducting portion extending longitudinally thereof, said wiper member having a guide slot therein, said housing having a longitudinal guide bar extending inwardly fro-m a wall thereof and being in slidable engagement with said guide slot, whereby said wiper member will be held against rotation and will move longitudinally relative to said mandrel upon an operative rotation of the mandrel, a continuous length of resistance wire releasably wound upon said mandrel member within portions of the threaded groove thereof in anking relation to said wiper member, la portion of said wire being raised above the mandrel and disposed over and in slidable engagement with a portion of said wiper member, whereby successive portions of said wire will be progressively elevated by the underlying portion of said wiper member during the latters operative movement relative to said mandrel, said wiper member having a portion thereof in slidable engagement with said conducting portion of said housing, and conductors electrically connected to said resistance wire and said conducting portion, respectively.

13. In a variable resistor, an externally threaded man drel member of insulating material, a centrally apertured internally threaded conductor Wiper member mounted upon and in threaded engagement with said mandrel, a housing for said mandrel and wiper member, said housing having a conducting portion extending longitudinally thereof, mounting means rotatably supporting said mandrel, means to hold said wiper member against rotation, whereby upon an operative rotation of said mandrel the wiper member will move longitudinally relative to the mandrel a continuous length of resistance wire releasably wound upon said mandrel member within portions of the threaded groove thereof in flanking relation to said wiper member, a portion of said wire being raised above the mandrel and disposed over and in slidable engagement with a portion of said wiper member, whereby successive portions of said wire will be progressively elevated by the underlying portion of said wiper member during the latters operative movement relative to said mandrel, said Wiper member having a portion thereof in slidable engagement with said conducting portion of said housing, front and rear shafts of conducting material extending axially from the respective front and rear portions of said mandrel, said shafts being rotatably supported by said mounting means, shaft contact members in engagement with said shafts, said shafts being electrically connected to the oppo site terminals of said resistance wire, and conductors electrically connected to said shaft contact members and said conducting portion, respectively.

14. In a variable resistor, the combination according to claim 13, an outer casing enclosing said housing, shafts, shaft contact members and conductors, and a rotatable cap operatively connected to one of said shafts.

References Cited in the le of this patent UNITED STATES PATENTS 2,229,072 Green Ian. 21, 1941 2,729,729 Knoll Jan. 3, 1956 FOREIGN PATENTS 606,457 Germany Dec. 3, 1934 778,594 France Sept. 19, 1934 598,122 Great Britain Feb. 11, 1948 708,386 Great Britain May 5, 1954 

